Saturday, 23 February 2013

Talk : Cochlea Ear Implants

Prof Palmer began by describing how remarkable an organ the ear is before moving onto the technology of cochlea implants and future developments in this area.

The Ear
There are three main parts to the ear. . .

The Human Ear

The Outer Ear comprises the visible ear and the ear canal, both of which gather and focus sound energy onto the eardrum, amplifying frequencies around 3kHz (the frequency of human speech) by some 10 times.

The Middle Ear comprises three tiny bones that transfer energy from the eardrum (that is vibrated by air pressure) to the inner ear (which is filled with salty water). In doing so the bones act as levers so the large displacement - low pressure vibrations of the eardrum is converted to a small displacement - high pressure vibration in the inner ear. This transformation is important as sound behaves differently in air and in water, Indeed, without the middle ear ossicles, only about 0.1 percent of sound energy would make it into the inner ear. The development of the bones of the middle ear is fascinating, with their original location being on the jaw of the growing embryo before they move to their final location later in the embyro’s development.

The Inner Ear is fluid filled and comprises a number of structures, of which the most important here is the Cochlea. This spiral structure contains a central membrane that runs almost all the whole way along it and it is on this membrane that the hair cells which actually detect sound are located. The membrane is thinner and stiffer at the entrance to the cochlea (so the membrane here resonates at higher frequencies) and less stiff towards the apex of the cochlea (so the membrane here resonates at lower frequencies

The Cochlea

Frog Hair Cells

The vibrations of the membrane bend the "hairs" of the hair cells mechanically opening channels in their structure which allow in charged ions from the surrounding liquid, and it is this which generates the electrical potential in the nerve fibres attached to the hair cells.

Ion Channels

History of electrical stimulation of the ear
Prof Palmer briefly covered the history of electrical stimulation of the inner ear by mentioning the experiments of Volta (~1780) and Simmins (1966) - both of which are covered here

The Cochlear Implant
For those who suffer profound deafness due to damage to the hair cells (either congenital or though illness/hearing damage/drug use) it may be possible to use a cochlear implant, which has the following key components :

Extenally, there is a microphone, a speech processor that splits sound into different channels and a transmitter to send the signals through the skin.

Internally there is a receiver and stimulator secured in bone beneath the skin, which converts the signals into electric impulses and sends them through an internal cable to an array of up to 22 electrodes wound through the cochlea and lying just under the central membrane. The electrodes stimulated the nerves coming out of the hair cells, which are stimulated to send impulses to the brain through the auditory nerve.

Schematic of a Cochlea Implant

During development of cochlea implants, there was some skepticism that would be effective, given that they only had a couple of dozen electrodes against the thousands of hair cells and that these electrodes only indirectly activated the hair cell nerves - and this seemed to be born out with the results from initial implants, which were given to adults who had become deaf at some point after learning language. In these cases the recipient often had difficulty adjusting to the implant.

Implants are now given to children as young as 10 months old. In many of these cases, the implant has given the child sufficient hearing to allow them to learn language, go to a standard school and live a very normal life, including participating in the job market on equal terms with the rest of the population. There are now some 100,000 in use worldwide, although they remain a technology that is much more prevalent in the developed world, given that the cost in the range of a few tens of thousands of pounds.

Prof Palmer mentioned how great work was being done by Nottingham based charity “The Ear Foundation” in treating people who had deafness - and also, critically, giving them support post-implant to ensure that they are able to adjust to the use of the implant as well as possible. As Prof Palmer explained, these implants can literally be life-changing to the recipient.

Other technologies
Finally, Prof Palmer gave a quick look at some of the technologies that are under development. Perhaps the most interesting of these is research into implants into the part of the brain that receives the auditory signals. These are currently only having partial success, but this is perhaps to be expected since the technology is at the same point on its development as cochlear implants were some 20 years ago.

Q&A
One of the best features of Café Sci is that the talks are short - but the Q&A session is long. Indeed, one can sometimes learn more from the Q&A than from the talk itself!

In this case, two things that came out of the discussion were comments describing how some in the deaf community viewed cochlear implants as a form of euthanasia and were very much against their use in small children. A view that BFTF finds difficult to understand.

And the other item was a suggestion to look up a video link on Youtube of a baby’s reaction on first having their new cochlea implant, which you can see below.